The present invention relates to the field of circulation control rotor (CCR) helicopters. Briefly, a CCR helicopter is one wherein the blades are rigidly fixed to the hub and lift is controlled by controlling the circulation of air around each blade (see U.S. Pat. No. 3,713,750). The primary advantage of a CCR helicopter is the reduction of weight of the entire rotor and reduction in number of moving parts associated with the blades; both of these result from the fact that the lift of each blade is modified by changing the circulation of air around it rather than by changing its angle of attack. The resulting rotor is lighter in weight, allowing a greater payload for the craft, and more reliable. CCR helicopters, however require a valve that will direct pulses of air into the rotor blades at a rate that does not vary appreciably with respect to the rate of rotation of the rotor. For this reason, most CCR valves are mechanically operated by a cam or other means on the rotor shaft; the air valves are thereby automatically opened at the proper azimuthal position of the rotor.
There are two main types of valves; those that are entirely within the hub, and those that are within the blades (except for the actuating mechanism, which is within the hub). Each type has advantages and disadvantages; a valve that is entirely within the hub is subjected to very low centrifugal forces and is quite compact, but this type of valve does not distribute lift control air (or other fluid) evenly along the length of the blade. That is, when a pulse of air is directed into the blade some of it immediately goes out through that part of the circulation control slot nearest the hub; this reduces the air pressure within the blade, so that less air goes out through the next part of the slot. The net result of this is an uneven airflow distribution pattern with a maximum at the hub and a minimum at the tip. A valve that extends the length of the slot in the blade solves the problem of uneven air distribution, but introduces problems due to centrifugal forces and to forces developed by unbalanced pressures on the operating part of the valve. An example of this latter would be a valve wherein the closure element is a flat plate that covers the circulation control slot along its entire length; the plate is hinged along one side, and is rotated about the hinge to uncover the slot. The unbalanced pressure force on this plate is the pressure on the inside (or plenum) surface of the plate minus the pressure on the outside (or slot) surface of the plate multiplied by the area of the slot; since the blade may be quite long, the area and hence unbalanced force may be quite large. What is needed, therefore, is a valve that extends the length of the blade but which is designed to overcome the problems due to unbalanced pressure forces on the closure element.